The conventional programmable voltage generator has a minimum voltage step of approximately 10-20 millivolts. Such a minimum voltage step is too large for voltage inputs to a temperature controller used with ovens for growing monocrystals.
Temperature controllers for ovens used in growing crystals may require temperature control of some tenths of a degree centigrade. A minimum voltage step of 1 millivolt or less typically would be needed to achieve a temperature variation of 0.1.degree. C. or less. Thus, the minimum voltage step of approximately 10-20 millivolts is a drawback against using a conventional programmable-voltage-generator to provide a voltage input to a temperature controller for ovens used in growing crystals.
One conventional programmable voltage generator is the Honeywell DCP 7700. Analagous generators are available from Leeds, Northrup, Eurothon and other companies. The oven generally used in the growth of monocrystals is the CZ type (Czochralski) oven. Horizontal Bridgman type ovens also are used. The growth technique generally used is the LEC technique.
The Honeywell programmable voltage ramp generator has the following characteristics: voltage outputs on 3 independent channels, capability of programming 99 temperature segments or settings, and storing of 9 programs. The nine programs define the voltage variation from an initial predetermined voltage value to a final value, while passing through intermediate values in established periods of time that are more or less prolonged.
A problem with the conventional generators is the characteristic of a minimum voltage step of approximately 10-20 millivolts. Such voltages correspond to minimum programmable temperature variations of approximately 1.degree. to 2.degree. C. In particular, such a minimum temperature variation results in a high number of lattice defects in the growth of a semiconductor type insulating or metallic crystal. These defects make the monocrystals obtained rather unsuitable for various applications, in particular for those in the electronics field (i.e. emitter diodes, microwave devices, lasers, integrated circuits). The Group III-V monocrystals (i.e. GaP and InP) result in a high dislocation density and often a high concentration of impurities. Such monocrystals are not stoichiometric and have a low structural homogeneity. Inclusions, microprecipitates, etc., may be present. This represents a significant drawback when the monocrystals are prepared by using a polycrystal as a precursor because the monocrystal will have a high number of dislocation and impurity defects.
Another drawback with a 1.degree. C. to 2.degree. C. (i.e. 10 mv to 20 mv) minimum step is that the high concentration of defects is also associated with a concentration of contaminating impurities which in turn generally contribute to the formation of lattice defects during the growth and cooling of the monocrystal.
To achieve a high perfection (instead of a high dislocation density) and a high purity (instead of impurity) in a monocrystal, it is necessary to have temperature variations down to 0.1.degree. C. or less in certain stages of the growth cycle.
Therefore, there is a need for special programmable generators capable of programming voltage variations of about 1 mV or less to enable the growth of monocrystals with a high perfection and purity.
By using the interface circuit of this invention with a conventional programmable voltage generator, it is possible to achieve such minimum voltage steps of 1 millivolt or less.